By press time, fierce fighting had continued in rebel-held districts of Aleppo for the second straight day as opposition forces continued to hold out in Syria's largest city.At least 200,000 residents of the country's commercial capital have fled in the past two days and many more are still trapped, according to the United Nations. The opposition Syrian National Council (SNC) on Sunday accused the government of preparing to carry out "massacres" in the northern city and pleaded for heavy weapons to enable rebels to meet the regime onslaught.The SNC also urged the UN to hold an emergency session to discuss ways to protect civilians caught up in the conflict.Several neighbourhoods in Aleppo where rebels are active have come under heavy government shelling, leading to high casualties among both fighters and civilians."Intense clashes took place in the districts of Bab al-Hadid, Zahraa, Arkub and Al-Hindrat Camp as explosions were heard and aircraft were sighted overhead," the Syrian Observatory for Human Rights said on Sunday.Helicopter gunships flew over the Saif al-Dawla district as well as Salaheddin, where rebels held off an offensive by ground troops backed by tanks and helicopters on Saturday, the Britain-based rights group said.Rebels claimed to destroy several government armoured vehicles, and American journalist Scott Peterson, a correspondent for the Christian Science Monitor, told Al Jazeera that fighters had been resupplied with rocket-propelled grenades and anti-aircraft guns."It was a very up and down fight at the very early stages," Peterson said. "I think [the victory] took the rebel forces by surprise a little bit. They claimed they knocked out eight tanks.""It was surprising to feel the degree of optimism that some of the rebels there felt," he said. "Of course what they were concerned about was that the quality and the volume of the shelling ... was going to yield kind of a destroyed neighbourhood." Other civilians in the city of some 2.5 million crowded into basements seeking refuge from the intense bombardment by artillery and helicopters, an AFP correspondent said.The London-based Observatory said more than 20,000 people, mostly civilians, have now died since the uprising against Assad's government erupted in March 2011."There are thousands of people in the streets fleeing the bombardment. They're being terrorised by helicopter gunships flying at low altitude," said an activist, adding many had taken refuge in public parks.'Causing exodus'Colonel Abdel Jabbar al-Oqaidi of the rebel Free Syrian Army said his forces on Saturday had repulsed troops in Salaheddin district and that the regime offensive had been temporarily stopped.